Often it is desirable to block light or any other optical band radiation from reaching a detector with the exception of radiation within a specific wavelength band. Narrow band optical systems have been developed for a wide variety of applications for many years. The usual objective is to isolate an optical signal of a specific wavelength in the presence of a large flux of noise, i.e. optical radiation at other wavelengths. A better signal to noise ratio can be derived from the detector if the signal is within a narrow band of the optical spectrum. Noise is caused by background light outside the band of the signal and such filters typically find use in wireless communication applications as described by Barry et al. in "High-Speed Nondirective Optic Communication for Wireless Networks", IEEE Network Magazine, November 1991.
In conventional filtering systems, the absorption of light at specific wavelengths is sometimes used as a means of filtering. However such filters are usually not capable of isolating a narrow passband, and they tend to be lossy at the desired band.
In a further approach, filters composed of layers of thin dielectric films consisting of materials with widely differing refractive indices have been utilized. Such filters are known as interference filters and are capable of isolating quite narrow passbands with relatively high transmission at the desired band. Interference filters, however, have the characteristic that the passband shifts as the angle the light ray makes with respect to the surface of the filter varies. This characteristic is a disadvantage in filtered optical systems which require wide angle reception again as described in Barry et al., supra.
A filtering system which provides for wide angle reception is described by Martin and Fohl in U.S. Pat. No. 5,124,859. Although this filter is a clear advance over the prior art, it can only operate at wavelengths for which suitable atomic transitions exist.
It is thus desirable to provide an optical filtering system that has a wide angle of acceptance but maintains the narrow passband of a interference filter normal to a well collimated beam. It is further desirable that such a filtering system be amenable to a simple and compact implementation.